1. Field of the Invention
This invention relates generally to a laser imaging and/or tracking system, and more specifically, to a laser imaging and/or tracking system incorporating speckle suppression.
2. Description of the Related Art
Coherence of a light beam can be simplistically described as the relationship between different phases of wavelets within the beam. If the phase difference between any pair of wavelets is a constant then the wavelets are perfectly coherent. Coherence can be separated into temporal and spatial coherence, both being a factor of overall beam coherence. Temporal coherence is related to the phase relationship between the various frequencies comprising the spectral bandwidth. The more monochromatic the beam is the greater the temporal coherence. Spatial coherence is related to the phase relationship between the various spatial locations of the wavefront comprising the beam. The higher the degree of transverse spatial coherence the less the beam diverges. While common luminous light sources are incoherent, the light beams emanating from lasers are highly temporally and spatially coherent. Coherent laser beams provide well known advantages for imaging and tracking systems due to the highly directional nature of the laser's propagation characteristics, resulting in efficient energy transfer of the output laser power to the surface of an illuminated target of interest. A basic problem with using highly coherent laser sources as illuminators for active tracking and/or imaging systems is the deleterious effects of random intensity variations, known as speckle, in the image. Speckle originates when a coherent light beam is reflected from or transmitted through an object having root-mean-square (rms) surface height deviations on the same order or scale as the light beam's wavelength. A speckle intensity pattern results at the image plane due to mutually coherent interference between reflected light from different spatial locations of the object. For typical imaging systems, the optical point spread function is large when compared to the surface roughness of the object. Consequently, the image intensity at a specific image point is the aggregation of coherent interference from many different object points. Since the random intensity pattern has a contrast ratio of approximately one (1), and the angular periods of the intensity fluctuations are on the order of the angular resolving power of the optical system, the effective pixel signal to noise ratio due to speckle is approximately one. Therefore, the suppression of speckle is a critical aspect of sensitive image and tracking devices.
One prior art method to suppress speckle in an active imaging system using a laser as an illuminator is the method of frame averaging. Frame averaging requires the acquisition of a plurality of image frames of the scene. For frame averaging to be effective each acquired image must measure a statistically different speckle pattern. The averaging process reduces speckle by approximately the square root of the number or frames, but also degrades image quality due to the induced image blurring caused by the relative target motion required by the frame averaging process. In addition, frame averaging impacts the power requirements for the laser, and eliminates the covert advantage of a single frame short pulse illumination.
What is needed then is a coherent light source imaging system which effectively suppresses speckle without effecting other critical system constraints. It is therefore an object of the present invention to provide such a system.